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We report the results of our first-principles calculations on the effect of isovalent, non-magnetic, Al 3þ ion doping on the electronic structure and spontaneous polarization of multiferroic BiFeO 3 . Our calculations reveal that Al 3þ doping in BiFeO 3 results in the reduction of Fe-O-Fe bond angle, leading to the weakening of antiferromagnetic superexchange interaction, further substantiated by the reduction of exchange interaction constant with increasing doping level. Lowering of welldepth is suggestive of reduced switching potential and improved P-E loop with lowered coercivity. Chemical bonding analysis by electron localization function shows that cation-oxygen bonding is of mixed ionic-covalent character, with marginal increase in the covalent character with increasing doping concentration. Large spontaneous polarization of undoped BiFeO 3 is retained with lower doping level (6.25%), while for higher doping content (31.25%), the spontaneous polarization is reduced, primarily due to larger c/a ratio at higher doping level. V C 2015 AIP Publishing LLC.

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We present a detailed theoretical study on electronic and magnetic properties of Mo nanowires with different structures. The ultrathin nanowires of this 4d transition metal show a unique behavior for the stability. We notice that zigzag structure is stable at the lower values of nearest neighbor distance. On slightly stretching the nanowire, the ladder structure is preferred while the dimerized structure, with the highest value of cohesive energy, is the most stable structure at larger nearest neighbor distances. This work suggests that magnetic ordering of Mo nanowires can be tuned with structure. The linear and ladder structures of Mo nanowires show antiferromagnetic ordering. Equilateral zigzag structure prefers a nonmagnetic state whereas the planar zigzag structure is ferromagnetic. The dimerized structure stands out showing degenerate nonmagnetic and ferromagnetic states. The highest value of magnetic moment (∼1.16 μB/atom) is predicted for linear chains. Relative break force values suggest that these nanowires would be difficult to be realized. The density of states and band structure shine light on engineering the electronic properties with structural tailoring. We notice that dimerized structure is the only one which can be used in semiconducting applications with a band gap of 1.1 eV. Interestingly, all these Mo nanowires show a signature of covalent bonding coexisting with metallic charge sharing, the former getting enhanced with the stretching of the wire.

We have investigated theoretically the electronic and optical properties of free-standing and substrate-supported ultrathin nanowires (NWs) of the transition metal vanadium. Ground state of the structures studied, except free-standing zigzag geometry, is found to be magnetic in nature. We show that for some structures, study of the antiferromagnetic state necessitates considering various possible configurations. All the structures, except dimerized, show metallic behavior. Structure with helical geometry possesses decent value of magnetic moment and is exceptionally stable as well as most stiff of all the structures studied. The plasma frequency and dielectric function nicely exhibit the anisotropy due to one-dimensional nature of the nanowires. The latter is structure-dependent and markedly different from that of bulk. More realistic case of linear chains supported on a substrate shows fair impact of the substrate in comparison with free-standing case. There is substantial charge redistribution on relaxing the geometry. The d-states are in general shifted to lower energies and the peaks in the density of states are broadened, resulting in softening of the structures in the optical spectra.